CN111125897A

CN111125897A - Fast calculation method for noise power ratio NPR of traveling wave tube

Info

Publication number: CN111125897A
Application number: CN201911300747.8A
Authority: CN
Inventors: 王小兵; 胡玉禄; 胡权; 朱小芳; 杨中海; 李斌
Original assignee: University of Electronic Science and Technology of China
Current assignee: University of Electronic Science and Technology of China
Priority date: 2019-12-17
Filing date: 2019-12-17
Publication date: 2020-05-08
Anticipated expiration: 2039-12-17
Also published as: CN111125897B

Abstract

The invention belongs to the technical field of traveling wave tube nonlinear distortion, and particularly relates to a method for rapidly calculating a Noise Power Ratio (NPR) of a traveling wave tube. Aiming at the difficulty in predicting the NPR (nonlinear design index) of the existing space traveling wave tube, the characteristic that the gain and the phase shift are unchanged in the calculation frequency range of the traveling wave tube is utilized, the input power is only required to be scanned once under a central frequency signal in a working bandwidth, then the output signal of the multi-frequency input signal under different input powers is rapidly solved by using a formula, the output signal is subjected to spectrum analysis, and the NPR under different input powers is obtained through calculation. The method effectively solves the problems of long calculation time and low efficiency of NPR (nonlinear programming) by utilizing the injection wave interaction simulation calculation, realizes the NPR prediction in the design stage of the traveling wave tube, effectively improves the nonlinear design and inhibition efficiency of the traveling wave tube, saves the design time and reduces the cost for developing the traveling wave tube.

Description

Fast calculation method for noise power ratio NPR of traveling wave tube

Technical Field

The invention belongs to the technical field of traveling wave tube nonlinear distortion, and particularly relates to a method for rapidly calculating a Noise Power Ratio (NPR) of a traveling wave tube.

Background

The traveling wave tube is a wide-band high-power vacuum electronic device and is widely applied to the fields of radar, communication, navigation and the like. The space traveling wave tube is widely applied to a communication satellite system by the characteristics of high power, high efficiency, ultra wide band, high reliability, long service life, radiation resistance and the like, and the space traveling wave tube is used as a final amplifier of satellite communication and plays an important role in signal amplification and transmission. In satellite communication, in order to reduce power consumption of satellite payload, under a limited available frequency bandwidth, a multi-carrier working mode is used to improve system data transmission capacity, and multi-channel communication needs to be realized through a single space traveling wave tube amplifier, that is, the working bandwidth of a traveling wave tube is divided into a plurality of channels, so that multiple signals can be transmitted simultaneously. Due to the influence of nonlinear distortion in the traveling wave tube, the concurrence of multiple signals in the communication process can cause mutual interference among the multiple signals, and the accuracy of satellite communication and data transmission is influenced. In order to ensure the communication quality, the space traveling wave tube is required to have good linearity. In order to reduce mutual crosstalk between signals and reduce the error rate of a system, a traditional method adopts a working point power backspacing mode to reduce nonlinear distortion influence, on one hand, the signal power influence communication distance is reduced, on the other hand, the working efficiency of a traveling wave tube is also reduced, and loss is caused to a communication satellite with a tense energy source. Therefore, the noise power ratio needs to be accurately calculated and analyzed during the design of the traveling wave tube.

In order to meet strict linearity requirements, the traditional linearity indexes such as amplitude modulation-amplitude modulation characteristics and amplitude modulation-phase modulation characteristics are not enough to accurately measure the linearity of the traveling wave tube in a multi-carrier working mode, and higher-level linearity indexes need to be specified. The Noise Power Ratio (NPR) is particularly suitable for analyzing the linearity characteristics of a traveling wave tube amplifier driven by a multi-carrier signal, and the criterion may take into account all types of interference, including noise, intermodulation, inter-channel interference and outer-channel interference. Evaluating NPR requires the input of a complex excitation signal. The signal is characterized by a white gaussian noise signal and has a portion of the spectrum removed (spectral notch). When the signal is input into the space traveling wave tube, the nonlinearity of the space traveling wave tube can cause the specific frequency spectrum component removed from the input signal spectrum to appear in the output signal. The noise power ratio is defined as the ratio of the power spectral density outside the output signal frequency spectrum depression to the power spectral density at the depression, and the fast and accurate prediction of the noise power ratio is always an important subject for designing the high-linearity space traveling wave tube.

The core of the traveling wave tube work is the interaction process of electron beams and electromagnetic waves: in the vacuum tube shell, a beam of electron beam generated from a cathode is transmitted in the same direction with the electromagnetic wave carrying frequency signals at a certain speed, in the process, the electrons are modulated by the electromagnetic wave, the energy of the electromagnetic wave is also excited to be amplified, finally, the electromagnetic wave signals are amplified, and the rest electron beam is collected by a collector. The process of energy exchange between the whole electron beam and the electromagnetic wave is called the process of wave interaction. At present, a Lagrange frequency domain nonlinear wave-injection interaction model is generally adopted in the wave-injection interaction simulation calculation of a traveling wave tube, and the model needs to adopt a plurality of limited macro-electrons to represent electronic states of different time phases in periodic time. The model can be used for quickly and accurately carrying out simulation calculation on the wave injection interaction. Usually, the simulation calculation time of the one-shot wave interaction process of the single-frequency signal is within tens of seconds, and the convergence can be realized by adopting the macroelectrons with 32 time phases, but the calculation accuracy can be realized only by generating extremely many macroelectrons along with the increase of the number of input signals, and the calculation of NPR requires inputting dozens of or even thousands of frequency signals, so that the calculation time is too long, the software calculation is broken down, and the calculation convergence problem exists.

At present, domestic fast calculation methods for space traveling wave tube noise power ratio simulation are less in research, the process of testing the noise power ratio of the space traveling wave tube is complex and takes too long, NPR of the tested traveling wave tube is tested after the traveling wave tube is manufactured, and at the moment, if the NPR does not meet the non-linear index required by a user, the traveling wave tube needs to be designed and manufactured again, so that the development cost of the traveling wave tube is greatly increased. Many domestic space traveling-wave tubes cannot meet the requirements of most users because they cannot meet strict non-linear indexes. Therefore, the NPR of the traveling wave tube cannot be predicted, the nonlinear design and the suppression of the traveling wave tube are restricted, the development cost of the traveling wave tube with high linearity is greatly increased, the development period is prolonged, and the performance of the space traveling wave tube is restricted.

Disclosure of Invention

The invention provides a fast calculation method for NPR of a traveling wave tube, aiming at the defects that the NPR of the existing nonlinear design index of the space traveling wave tube is difficult to predict and the development of the high-linearity traveling wave tube is seriously restricted. The method utilizes the characteristic that the gain and the phase shift are not changed in the frequency range calculated by the traveling wave tube, only needs to scan the input power once under the central frequency signal in the working bandwidth, then utilizes a formula to quickly solve the output signal of the multi-frequency input signal under different input powers, carries out spectrum analysis on the output signal, and calculates to obtain the NPR under different input powers.

In order to achieve the purpose, the invention adopts the technical scheme that:

the method comprises the following steps: constructing the input signal required for calculating NPR, the input signal being n_fA frequency signal n_fAnd > 10, and the frequency interval between two adjacent frequency signals is equal. The center frequency of the input signal is f₀. Setting the amplitude of each frequency signal to accord with the Gaussian distribution with the mean value of 0 and the variance of 1, and the phase to accord with the value of [0,2 pi]Random and uniform distribution within. At a central frequency f₀Setting the amplitude of 2-5 frequency points to 0 (frequency notch) for the center, with a given total input power P_inCalculating n_fThe amplitude corresponding to each frequency signal. N is a_fThe frequency signal being characterised by a centre frequency f₀A single frequency signal.

Step two: at a central frequency f₀And as the input signal frequency, scanning the input power by taking the small signal power of 20-25dB of saturated input power back-off as a starting point and taking the saturated input power plus 3-5dB as an end point, and acquiring the scanned gain curve G and the phase shift curve phi data. Thereby obtaining the center frequency f₀Complex gain curve data at different input powers of the signal.

Step three: n is_fThe frequency signal being regarded as the central frequency f of the amplitude variation₀Using the single frequency signal of step two₀Calculating the output of the multi-frequency input signal by the complex gain curve data of the single-frequency signal under different input powersA signal. And carrying out Fourier transform on the output signal to obtain an output signal frequency spectrum. Accumulating M times of input signals, averaging the frequency spectrum data of the M times of calculated output signals, and finally calculating the averaged frequency spectrum to obtain the total input power P_inNPR of (ii).

Step four: varying the total input power P_inAnd repeating the process of the third step to obtain different input powers (namely different P)_inValue) NPR.

The method for rapidly calculating the NPR of the traveling wave tube effectively solves the problems of long calculation time and low efficiency of NPR calculation by utilizing the wave injection interaction simulation, solves the problem of difficulty in predicting the NPR of the traveling wave tube in the design stage, realizes NPR prediction in the design stage of the traveling wave tube, effectively improves the efficiency of nonlinear design and inhibition of the traveling wave tube, saves the design time and reduces the cost for developing the traveling wave tube.

Drawings

FIG. 1 is an input signal spectrum of an embodiment;

FIG. 2 is a graph of gain and phase shift for implementing a routine wave tube input power sweep;

FIG. 3 is a frequency spectrum of an input signal and an output signal of an embodiment;

FIG. 4 is a schematic illustration of an NPR of an embodiment;

FIG. 5 shows NPR calculation and test comparison of Ku band spatial traveling-wave tubes of examples.

Detailed Description

The present invention is further described in detail below with reference to mathematical model formulas and the accompanying drawings.

Description of the first step:

constructing the input signal required for calculating NPR, the input signal being n_f(more than 10) frequency signals, and the frequency interval between two adjacent frequency signals is equal. The center frequency of the input signal is f₀. Setting the amplitude of each frequency signal to accord with the Gaussian distribution with the mean value of 0 and the variance of 1, and the phase to accord with the value of [0,2 pi]Random and uniform distribution within. At a central frequency f₀Setting the amplitude of 2-5 frequency points to 0 (frequency notch) for the center, with a given total input power P_inCalculating n_fThe amplitude corresponding to each frequency signal. N is a_fThe frequency signal being characterised by an intermediate frequency f₀A single frequency signal.

The input signal required for calculating NPR is constructed, as shown in equation (1), input signal U_inIs n_fA frequency signal with a minimum frequency of f_minMaximum frequency of f_maxFrequency spacing between two adjacent frequency signals

Are equal. Calculated center frequency f₀＝(f_min+f_max)/2，U_kIs the amplitude of the k-th frequency signal,

is the initial phase of the k-th frequency signal. t is time. Setting the amplitude of each frequency component of the input signal to conform to a Gaussian distribution with a mean of 0 and a variance of 1 to generate M × n_fA random number P of Gaussian distribution_k(k＝1,2,...,n_f). The phase is coincident with [0,2 pi ]]Random uniform distribution of inner, generating Mxn_fUniformly distributed random numbers

At a central frequency f₀The 3 frequency points with amplitude 0 (frequency notch) are set for the center as shown in FIG. 1 (center frequency f)₀＝1.5GHz，n_f13 frequency signals, P_inAt-5.5 dB) spectrum has a frequency bin signal power of 0 around the center frequency, as shown by the frequency notch around the center frequency in the input spectrum in fig. 1.

Setting a shape factor s_k(k＝1,2,...,n_f) Generating pits in the middle of the pass band and pits at the bottom s_k0 at the edge of the pit

Providing s outside the pit_k1 at f_maxAnd f_minSetting s_k＝0。

Calculating n_fA frequency signalThe total input power of the number is P_inAs shown in equation (2). According to n_fTotal input power P of individual frequency signals_inThe amplitude U corresponding to each frequency signal can be calculated_kAs shown in equation (3).

Corresponding to the amplitude of the kth frequency component, where p_k| is an absolute value of a gaussian-distributed random number corresponding to the kth frequency component, and the normalization factor:

e.g. calculated at frequency f₀1.5GHz as the center frequency, n_fInput total power P as 13 frequency signals_inThe spectrum of the input signal at-5.5 dB is shown in figure 1.

The input signal needs to be calculated for many times, the amplitude of each frequency of the input signal is a random number conforming to Gaussian distribution each time, and the phase is a random number conforming to uniform distribution. The input signal is different for each calculation. Calculating the input signal M times, so it is necessary to generate M × n_fA random number of amplitude and phase.

N is a_fThe individual frequency signals are characterized as center frequency single frequency signals, as shown in equation (4).

Whereint is time, f₀Is the center frequency, omega₀＝2πf₀Is the center angular frequency, f_kFor the frequency corresponding to the kth frequency signal, Δ ω_k＝2π(f_k-f₀) Is the difference, U, of the kth frequency signal and the central angular frequency signal_kIs the amplitude of the k-th signal,

is the initial phase of the kth signal.

The second step is explained as follows:

with n_fCenter frequency f of frequency signal₀And as the input signal frequency, scanning the input power by taking the small signal power of 20-25dB of saturated input power back-off as a starting point and taking the saturated input power plus 3-5dB as an end point, and acquiring the scanned gain curve G and the phase shift curve phi data. Thereby obtaining the center frequency f₀Complex gain data at different input powers of the signal.

For example, input n_fA frequency signal, lowest frequency f_min1.494GHz, maximum frequency f_max1.506GHz, the center frequency is f₀1.5 GHz. And the saturated input power is-2.5 dBm, the small signal power after 20dB backoff is-22.5 dBm, and a gain curve G (a gain curve in fig. 2) and a phase shift curve Φ (a phase shift curve in fig. 2) are obtained.

Complex gain g (P)_in) Denoted by G and φ as:

G(P_in) For an input power of P_inGain of time, [ phi ] (P)_in) For an input power of P_inPhase shift of time, j being the unit of imaginary number, g (P)_in) Representing input power as P_inThe complex gain of time.

The third step is explained as follows:

n_ffrequency of eachThe rate signal being regarded as the center frequency f of the amplitude variation₀Using the center frequency f obtained in step two₀The output signal of the multi-frequency signal can be calculated according to the complex gain curve data of the signal under different input powers. And carrying out Fourier transform on the output signal to obtain an output signal frequency spectrum. Accumulating M times of input signals, averaging the frequency spectrum data of the M times of calculated output signals, and finally calculating the averaged frequency spectrum to obtain the total input power P_inNPR of (ii).

As can be seen from equation (4), the multi-frequency input signal can be regarded as the intermediate frequency f with amplitude variation₀The input signal is amplified by the traveling wave tube through the interaction of the injection waves. Using the center frequency f obtained in step two₀Obtaining the output signal U of the multi-frequency input signal by using the formula (6) according to the complex gain curve data of the signal under different input powers_out(t) of (d). To output signal U_out(t) performing Fourier transform to obtain an output signal spectrum. Accumulating and calculating M times of input signals, averaging the frequency spectrum data of the output signals calculated by M times, and finally calculating the averaged frequency spectrum to obtain the input power P_inNPR of (ii).

U_out(t)＝g(|C(t)|²)C(t)cos(2πf₀t-φ(t)) (6)

In the formula (6), C (t) represents the center frequency f for the input multi-frequency signal₀The amplitude after the signal. I C (t)²I is the representation of the input multi-frequency signal as the central frequency f₀Instantaneous power after the signal. g (| C (t))²) Having a central frequency signal input power of | C (t)²Complex gain function at l.

For example, by using measured gain and phase shift data of a certain Ku band space traveling wave tube, the number of input frequencies is set as follows: n is_fAdjacent frequencies are equally spaced at 1MHz, with the total input power being-3 dB relative to the saturated input power. The calculated M is 1000 times the input signal, the output signal is subjected to spectral analysis, the spectral data is averaged, the obtained input signal and output signal spectra are shown in fig. 3, and the calculated NPR is shown in fig. 4.

Step four is explained as follows:

varying the total input power P_inAnd repeating the process of the third step to obtain the NPR under different input powers.

FIG. 5 is a comparison between NPR calculation and test of a Ku-band space traveling wave tube, which verifies the accuracy of the calculation method of the invention, and the error between the calculation and the test data is within 1.1 dB.

In conclusion, the method effectively solves the problems of long calculation time and low efficiency of NPR (nonlinear numerical control) calculated by utilizing the wave injection interaction simulation; the method is applied to the traveling wave tube design stage to predict the noise power ratio of the traveling wave tube, solves the problem that the NPR of the traveling wave tube is difficult to predict in the design stage, realizes the NPR prediction in the design stage of the traveling wave tube, effectively improves the efficiency of nonlinear design and suppression of the traveling wave tube, saves the design time, and reduces the cost of developing the traveling wave tube.

Claims

1. A fast calculation method for an NPR of a traveling wave tube comprises the following specific steps:

the method comprises the following steps: the input signals required for calculating the NPR are constructed:

input signal is n_fFrequency signals, the frequency interval between two adjacent frequency signals is equal, the center frequency of the input signal is f₀Setting the amplitude of each frequency signal to accord with the Gaussian distribution with the mean value of 0 and the variance of 1, and the phase to accord with the value of [0,2 pi]Random uniform distribution of inner, n_fIs more than 10; at a central frequency f₀Setting the amplitude of 2-5 frequency points as 0, i.e. frequency notch, for the center, by a given total input power P_inCalculating n_fAmplitude corresponding to frequency signal, n_fThe frequency signal being characterised by a centre frequency f₀A single frequency signal;

step two: at a central frequency f₀As the input signal frequency, the input power scanning is carried out by taking the small signal power of 20-25dB of saturated input power back-off as a starting point and taking the saturated input power plus 3-5dB as an end point, and the data of a scanned gain curve G and a phase shift curve phi are obtained, so that the central frequency f is obtained₀Complex gain curve data at different input powers of the signal;

step three: using f obtained in step two₀Calculating the output signal of the multi-frequency input signal by the complex gain curve data of the single-frequency signal under different input powers, and performing Fourier transform on the output signal to obtain an output signal frequency spectrum; accumulating M times of input signals, averaging the frequency spectrum data of the M times of calculated output signals, and finally calculating the averaged frequency spectrum to obtain the total input power P_inNPR of (ii).

Step four: varying the total input power P_inAnd repeating the process of the third step to obtain different input powers, namely different P_inNPR at value.

2. The fast NPR calculation method for the traveling wave tube according to claim 1, applied to the traveling wave tube design stage to predict the noise power ratio of the traveling wave tube.